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Re: [Libvir] [PATCH] virDomainMigrate version 4 (for discussion only!)



Some observations about Xen migration and error handling.

The Xen migration protocol isn't stable between releases. It changed between 3.0.3 and 3.1.0. There doesn't seem to be any versioning, and incompatible versions of Xen seem happy to attempt migrations between them, even though these will certainly fail.

The source host's xend forks an xc_save process. It appears to me that xc_save will happily write to _anything_ listening on port 8002, even if that thing closes the socket prematurely. (Try running 'xm migrate' on one host and at the same time 'nc -l 8002 > /dev/null' on the target host. The 'xm migrate' will happily complete without error. Meanwhile the domain you "migrated" just gets deleted.)

Partly because of the lack of error reporting, and partly because the xend -> xc_save fork will make error reporting difficult to add, libvirt has a hard time displaying errors in this situation. It is quite possible to call virDomainMigrate and get a domain back which shortly afterwards "disappears", all without any indication of error.

Rich.

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